Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Sep. 30, 2014
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	$ 3.3		$ 3.3		$ 5.5
Change in fair value of contingent consideration for acquisitions	0.2	$ (0.2)	1.9	$ 0.6
Tradewire Acquisition [Member] [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	1.8		1.8
Present Value of Estimated Total Purchase Price	3.9		3.9
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	1.5		1.5
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	1.5		1.5
Present Value of Estimated Total Purchase Price	28.9		28.9
Contingent Consideration [Member]
Business Acquisition, Contingent Consideration [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	$ 0.2	$ (0.2)	0.4	0.1
Change in fair value of contingent consideration for acquisitions			$ 0.4	$ 0.1